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Collection of Elements

©2000 Timothy Paustian, University of Wisconsin-Madison

All organisms require very similar elements when making more of themselves. The exact requirements of microbes are discussed in detail in the section on Nutrition and Growth of Bacteria. Microbes obtain these basic building blocks from their environment. Most elements are not found free in the environment, but are part of molecules - Nitrogen is often found as NO₃^-2; Carbon as sugars, amino acids and CO₂, Sulfur as SO₄^-2 or H₂S and so on. Microbes must first find these molecules in their environment and then collect them inside the cell.

Finding food

Many microbes are non-motile and are prisoners of chance, being subject to greater forces around them that bring food their way or not. Their's is a feast of famine existence and they are very good at surviving lean times until suitable nutrient conditions present themselves. Other microbes are motile and have more control over there nutritional circumstance. Often these motile microbes will exhibit a type of behavior know as chemotaxis. Food sources serve as attractants and the bacteria will move towards them, eventually congregating in an area high in needed nutrients.

Transport

Once the necessary nutrients are available in the environment the bacteria must bring them inside the cell to be available for biosynthesis. This process is known as transport and in most cases requires energy because the nutrients are needed in concentrations much higher than what is available outside the cell. Transport of molecules involves two separate processes. Recognition of the desired molecule and then translocation across the membrane.

Recognition

Desired molecules normally interact with recognition proteins (receptors) on the outside surface of the cellular membrane. (Remember that the other parts of the cell wall, the outer membrane in gram negative bacteria and the peptidoglycan in both gram positive and gram negative bacteria, are porous and do not prevent small molecules from reaching the cytoplasmic membrane.) These receptors can be binding proteins that float in the periplasm just outside the cell membrane, or they can be part of transport proteins that span the membrane into the cytoplasm. In either case, there is a specific interaction between the nutrient that is recognized and a protein designed to bind that molecule. Free floating binding proteins that have a nutrient in tow will then migrate and bind to a transport protein.

Translocation

Once a nutrient interacts with a transport protein either directly of through a binding protein it is then moved inside the cell. To achieve this concentration against a gradient energy is expended in the form of ATP or proton motive force. Once inside, the molecule is ready for biosynthesis. The process of transport is described in detail in the section on membranes.

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